I'd be sure Jim's idea isn't the case. But inevitably when I take my coach "out of storage" the water pump takes forever to prime up. Have yet to find a solution other than patience. A slight air leak due to a dry seal somewhere could also cause it. Check all fittings between tank, filter, and pump inlet.
Not sure what you mean... you're not hearing the pump at all, or just not its bogging down under moving water? If the switch lights but no pump motor period, then check pump fuse and connections at the pump.
Joel